It is known to provide a tube-type heat exchanger or cooler for the cooling of gases, such as gases which contain SO.sub.2, the cooler or heat exchanger comprising an array of mutually parallel spaced-apart lead tubes which are externally cooled and span a pair of spaced-apart tube sheets.
In the heat-exchanger or cooler housing, a cooling fluid, e.g. water under pressure, can be introduced into the space around and between the tubes while the tubes themselves communicate with an inlet and an outlet for the gas stream, the inlet and outlet sides having diffuser configurations to distribute and collect the gas.
Various limiting conditions must be taken into consideration in the design of such gas coolers. For example, it is desirable to provide large units so that the system will have low gas-pressure drop and flow resistance and a maximum throughput and cooling efficiency.
However, this desire is opposed by the fact that the tubes are composed of lead and hence the cooler consists, in large measure, of lead so that the cooler is extremely heavy, especially if it is large.
Thus the size of the cooler has been limited by the ability to economically manufacture, transport and install same with the aid of conventional hoisting and transport equipment so that special apparatus for handling the unit is not required.
Therefore it is desirable, for a given size of gas cooler, to reduce the amount of lead contained therein. This is accomplished by using lead tubes having the smallest possible wall thickness. Especially thin lead tubes are thus preferred in practice.
This, however, gives rise to another problem. The operating conditions (e.g. pressure) and low strength of the lead impose a lower limit on the wall thickness for a given service life. Consequently, attempts have been made to improve upon the strength of the lead-containing material itself by alloying techniques. Even this cannot be carried out to the extent necessary to maximize the strength of thin-walled lead tubes because erosion resistance is reduced with many alloying systems.
As a consequence, the prior-art systems using lead-tube heat exchangers or coolers have always been a compromise in consideration of all of the factors enumerated above. It is conventional in the art, therefore, to provide an extruded lead tube of a cylindrical outer configuration with an outer diameter of 70 mm and a wall thickness of about 6 mm, the tube wall having internal longitudinally extending fins or ribs. These fins may have a radial dimension of say 13 mm and can be provided in various heights, angular spacing and number. For example, six thin fins 13 mm high can be interleaved with six smaller intervening fins of a height of 7 mm in a particular tube construction.
Experiments have been made in attempts to reduce the quantity of lead per tube without materially reducing the flow cross section. For example, when an internally finned tube of the last-mentioned type but with an outer diameter of only 68 mm and a wall thickness of 5 mm was tested, with a saving of about 15% in material and weight, it proved unsatisfactory when exposed to normal operating conditions for prolonged periods.
Within about three hours of operation, each tube assumed gradually an oval shape and subsequently completely collapsed until the fins contacted each other.
Measurements have confirmed that the deformation of the tubes under operating conditions always begins at portions of the tube wall at which thickness is at the lowest tolerance limit.
Experience has indicated that conventional extrusion techniques cannot avoid eccentricities in the die core so that deviations of the wall thickness amounting to .+-.5% along the length of the tube and from tube to tube are not unusual.
The total or partial collapse of even some of the tubes of a gas cooler or hear exchanger renders the latter ineffective because the distribution of the gas and the cooling effect no longer are uniform and predictable in view of the reduced cross section of certain tubes.
Obviously, therefore, it is not a solution to simply attempt to reduce the wall thickness and outer diameter to decrease the weight.
Mention should be made of the fact that the collapse occurs, in conventional tubes, notwithstanding the expected reinforcing effect of the longitudinally extending fins or ribs which would suggest to the field that there is no solution to be found in modifying the configurations of the tubes for stiffening purposes.